If you have your Notifications set up correctly as the page Yasaf links says, there have been instances where mucking around with the mail settings (incoming and outgoing server) can result in notifications not going.

Try clearing all your mail settings and save the null settings, then re-enter the settings again. Sometimes helps to restart Spiceworks when you save the cleared settings, before you re-enter them.

If you are using Exchange settings, it sometimes also helps to use MAPI or POP and SMTP instead.